feat(wip): implement trip persistence through a local repository. Include loaded trips in the start page UI

frontend/lib/domain/repositories/trip_repository.dart

@@ -6,4 +6,14 @@ abstract class TripRepository {
   Future<Trip> getTrip({Preferences? preferences, String? tripUUID, List<Landmark>? landmarks});
 
   Future<List<Landmark>> searchLandmarks(Preferences preferences);
+
+  // TODO - should these be moved to a separate local repository?
+  // not every TripRepository should have a concept of "all saved trips"
+  Future<List<Trip>> getSavedTrips();
+
+  Future<Trip?> getSavedTrip(String uuid);
+
+  Future<void> saveTrip(Trip trip);
+
+  Future<void> deleteSavedTrip(String uuid);
 }